1. Field of the Invention
The present invention relates to a synthetic resin emulsion for an easily water-swellable pressure-sensitive adhesive, an easily water-swellable pressure-sensitive adhesive composition comprising the same, and a process for producing the synthetic resin emulsion. The easily water-swellable pressure-sensitive adhesive composition is suitable for applications such as labels, tapes, building materials, packaging materials, and electronic materials.
2. Background Art
Elastic bodies such as natural rubbers and synthetic rubbers or acrylic resins have generally hitherto been used as components of pressure-sensitive adhesives. In the formation of pressure-sensitive adhesive labels, sheets, and tapes, these components are applied onto the surface of a support. Such pressure-sensitive adhesive labels, sheets, and tapes can be easily bonded to the surface of an object at room temperature by pressure around a finger pressure and thus have become extensively used in various applications.
For example, these pressure-sensitive adhesive labels, tapes and the like are applied to adherends such as bottles, cans, or plastics. After use, these adherends are often corrected and recycled. The pressure-sensitive adhesive labels and the like applied onto the adherends, however, may become an obstacle to recycling of the adherends in a recycling process.
In some applications, after the pressure-sensitive adhesive label, tape or the like is temporarily used, the label, tape or the like is desired to be removed at once from the adherend. For example, in polishing of the backside of a silicon wafer as an electronic material, a pressure-sensitive adhesive sheet is used for supporting the wafer and protecting a surface pattern. This pressure-sensitive adhesive sheet is separated from the wafer after the polishing of the wafer. In this case, that the pressure-sensitive adhesive does not remain unremoved on the surface pattern is required.
Pressure-sensitive adhesive sheet are also used in sanitary goods. In this application, easy treatment of filth by water washing is required of the pressure-sensitive adhesive sheet.
In recent years, various alkali-swellable pressure-sensitive adhesives in which the pressure-sensitive adhesive can be dispersed or dissolved by alkaline water treatment in the recycling process have been put to practical use. The alkaline water treatment necessary for this treatment of the pressure-sensitive adhesive, however, possibly has an adverse effect on environment. Further, in pressure-sensitive adhesive sheets for electronic materials and sanitary goods, such alkali treatment per se cannot be carried out without difficulties.
For this reason, the development of a pressure-sensitive adhesive, which can easily be separated from the adherend with the aid of water and can be dissolved or dispersed in water without use of any alkali or acid, has been desired. This pressure-sensitive adhesive is preferably an aqueous pressure-sensitive adhesive also from the viewpoint of an environmental problem.
Synthetic resin emulsions produced by polymerizing acrylic monomers or the like in the presence of an emulsifier have hitherto been extensively used as the aqueous pressure-sensitive adhesive. By virtue of a relatively large molecular weight of the polymer, such synthetic resin emulsions can exhibit good water resistance in aqueous pressure-sensitive adhesives. Due to this nature, disadvantageously, the dried film is not easily swollen with water. For this reason, in order to render the polymer in this synthetic resin emulsion swellable with water, it is common practice to use a large amount of a hydrophilic surfactant or to add a water-soluble polymer such as polyvinyl alcohol or cellulose ether.
The use of a large amount of the surfactant sometimes causes plasticization of the polymer with the surfactant and consequently, however, disadvantageously causes a change in adhesive properties with the elapse of time. Further, the use of a large amount of the water-soluble polymer sometimes enhances the viscosity of the pressure-sensitive adhesive, deteriorates the storage stability due to the function of the water-soluble polymer as a coagulating agent, or deteriorates coatability due to structural viscosity.
On the other hand, a method, in which a polymer having a high carboxyl group content produced by solution polymerization is brought to a water-soluble polymer using an aqueous alkaline solution and emulsion polymerization is carried out using the water-soluble polymer as an emulsifier, has also been extensively used.
The polymer thus obtained, however, is swellable with water only under alkaline conditions. Further, in the polymerization process, after solution polymerization, the polymer is brought to an aqueous alkali solution followed by emulsion polymerization. This operation is troublesome. Further, this method suffers from an additional problem that the solvent used in the solution polymerization stays in the synthetic resin emulsion.
Accordingly, the development of a pressure-sensitive adhesive composition, which has excellent basic adhesive properties such as adhesion, cohesive force, and tackiness required of the pressure-sensitive adhesive and, at the same time, can be easily swollen with water without any alkali treatment, has been desired.